1. Field of Invention
The present invention relates to a control system for a scanner. More particularly, the present invention relates to a control system for controlling the charge coupled device (CCD) of a scanner.
2. Description of Related Art
The color charge coupled device (CCD) of most color scanners often includes special detection device for extracting the three primary colors R, G, B (red, green and blue) in pixels. As the CCD detects the brightness level of each primary color, different amount of charges will be accumulated depending on intensity of the color. After a period of exposure, the potential resulting from the accumulation of charges in each light-sensing point is transferred out to create a section of analog signal. The whole process, starting from the shining of a beam of light on the CCD to the generation of an analog signal, must go through a series of steps including light exposure, charge transfer and voltage shifting. At present, the CCD inside most color scanners has pixel detection units for the three primary colors red R, green G and blue B, and transfer gates and shift registers for each primary color. After moving the accumulated charges from the transfer gate to the shift register, an output analog voltage is generated.
Conventionally, there are several methods for detecting and acquiring information about the brightness level of a color. One method is to tap into the analog signals obtained through the conversion of primary color intensity level during each pixel clock cycle. FIG. 1A is a timing diagram showing a conventional method of acquiring the brightness intensity of all three primary colors and converting the intensities of the primary colors into analog signals. As shown in FIG. 1A, in each section of the pixel clock (between two analog signals), the accumulated charges resulting from the intensity levels of three primary colors are transferred to the shift registers via the transfer gates simultaneously. Thereafter, the analog signals that represents the intensity levels of the three primary colors RGB are converted to digital signals through an analog-to-digital converter (ADC). Although the method can provide a faster reading speed, analog-to-digital conversion of all three primary colors must be completed within each transfer period. Hence, clocking frequency of analog-to-digital conversion must be three times as fast as the frequency needed to capture the analog signals. Consequently, a structurally complicated and fast analog-to-digital converter is required to operate in this mode.
FIG. 1B is a timing diagram showing a second conventional method of acquiring the intensity of all three primary colors and converting the intensities of the primary colors into analog signals. In this mode of operation, the analog signal of only one of the three primary colors is captured in each pixel clock period. Since the CCD only provides a single transfer control signal line, the acquisition of analog signals that represents the intensity levels of the three primary colors must be displaced and positioned after the transfer clock pulse. Because only one transfer signal is allowed, a sufficiently bright light source must be provided by the scanner when signals are shifted out so that a sharp color contrast is obtained. In order to provide sufficient light intensity, a lot of electric power is wasted. However, if insufficient light is supplied, frequency of shift pulses must be reduced to increase the amount of exposure. Hence, scanning speed will drop. In other words, it is impossible to lower electricity consumption and increase operating speed without lowering the quality of scanned image in this mode.
In a third conventional method, the charge coupled device is capable of providing three transfer control signal lines. Hence, different time periods can be used for transferring the intensity of each primary color and exposure time can be extended without reducing scanning speed. FIG. 1C is a timing diagram showing the third conventional method of acquiring the intensity of all three primary colors and converting the intensities of the primary colors into analog signals. Although better scanning quality can be obtained without using too bright a light source, the method demands a continuous movement of the motor when the intensity level of a first primary color is transferred. One consequence of such movement is a vertical shift in the first primary color that may cause a mismatch with the other two primary colors. Therefore, a portion of the scanned image may be blurred. For example, in a black and white document, colored image data may emerge due to a color mismatch.
In brief, the conventional methods of controlling color scanning has several drawbacks, including:
1. If the analog signals representing all three primary colors are extracted within each pixel clock, image data can be read a lot faster. However, structurally complex and fast analog-to-digital converter must be used.
2. If the analog signal of one of the three primary colors is captured in each pixel clock period, the scanner must consume lots of electricity or else the scanning speed is slow. This is because only a single transfer control line is provided by the charge coupled device. It is impossible to lower electricity consumption and increase operating speed without lowering the quality of scanned image at the same time.
3. If the charge coupled device provides three transfer control signal lines, better scanning quality can be obtained without using too bright a light source. However, a vertical shift in color matching may occur leading a blur image.